“Plant growth can have a considerable
effect on the climate,” says Wolfgang Buermann, a geographer at Boston
University. He explains that there are several ways in which plants can alter
the temperature of the Earth’s atmosphere. Through the process of
photosynthesis, plants use energy from the sun to draw down carbon dioxide from
the atmosphere and then use it to create the carbohydrates they need to grow.
Since carbon dioxide is one of the most abundant greenhouse gases, the removal
of the gas from the atmosphere may temper the warming of our planet as a whole.

Plants also cool the landscape directly through the process known as
transpiration. When the surrounding atmosphere heats up, plants will often
release excess water into the air from their leaves. By releasing evaporated
water, plants cool themselves and the surrounding environment. “It’s
like sweating. When you sweat you cool the surface of your skin,” says
Buermann. Over a forest canopy or a vast expanse of grassland, large amounts of
transpiration can markedly increase water vapor in the atmosphere, causing more
precipitation and cloud cover in an area. The additional cloud cover often
reinforces the cooling by blocking sunlight.

Because of these processes, many researchers believe plants may have a
sizable impact on global climate in the future. As humans continue to generate
carbon dioxide and other greenhouse gases, the Earth’s surface will likely
warm at a faster rate than it has in a thousand years. According to the
Intergovernmental Panel on Climate Change (IPCC), the Earth is likely to warm
another 1.4 degrees to 5.8 degrees by the end of this century (IPCC 2001).
Needless to say, such big changes in the climate would likely alter vegetation
growth all over the world. Many researchers hypothesize that the changes in
vegetation could either serve to worsen or put a damper on global warming. If,
for instance, the increased temperature and carbon dioxide levels of the Earth
cause vegetation worldwide to flourish, plants could draw down more carbon
dioxide and thus reduce the impact of the greenhouse effect. If, on the other
hand, global warming causes widespread drought, then the loss of vegetation may
result in even higher surface temperatures.

To model and then understand the ways in which vegetation interacts with the
climate, scientists will need to maintain an accurate record of the
Earth’s vegetation well into the future. For this purpose, for roughly the
past twenty years, researchers have employed multi-spectral remote sensing
satellite instruments such as the Advanced Very High Resolution Radiometer
(AVHRR) instrument aboard NOAA’s polar-orbiting satellites. As is the case
with most remote sensing satellite instruments, AVHRR houses a number of
separate types of light detectors, which acquire images of different bands
(colors) of light reflected off of or emitted from the Earth’s surface and
atmosphere, including blue, green, red, near-infrared, and even thermal infrared
energy. From these satellite data, scientists can produce images of the Earth
showing a single band of light or a combination of bands. With a resolution on
the order of 1 square kilometer per pixel and up, AVHRR images are not well
suited for viewing details of the planet’s surface any smaller than a
farm, but they are extremely useful for mapping and monitoring vegetation on a
global scale.

As plants ‘breathe’ and ‘perspire’ they help
cool the atmosphere. Plants consume carbon dioxide—a significant greenhouse
gas—in the process of photosynthesis. The reduction of carbon dioxide in the
atmosphere has an indirect cooling effect. Plants also cool the atmosphere because they release water vapor
when they get hot, a process similar to sweating. The diagram at left shows the microscopic structure of a leaf, and the processes of photosynthesis and
transpiration. (Illustration courtesy P.J. Sellers et al.)

The amount and extent of vegetation, however,
cannot be discerned from the raw satellite images alone. To extract information
about vegetation from the satellite data, scientists must manipulate the images.
The preferred method for years has been the normalized difference vegetation
index (NDVI). Developed in 1979 by a NASA researcher, NDVI is a measure of the
green, leafy vegetation density or the lushness of vegetation. [For more details
see Measuring
Vegetation (NDVI & EVI)] NDVI is
produced by observing the discrepancy between the visible and near-infrared
sunlight that reflects off of vegetation. As can be seen through a prism, many
different wavelengths make up the spectrum of sunlight. The pigment in plant
leaves, chlorophyll, strongly absorbs the visible light in the solar spectrum
for use in photosynthesis. The cell structure of the leaves, on the other hand,
strongly reflects near-infrared solar light. By measuring the difference between
these two wavelengths of light in remote sensing data, scientists can get a
relative measure of vegetation. If the difference is large, an area is likely to
be densely vegetated, and if the value is small, the vegetation is likely to be
sparse.

The graph at left shows how efficiently the
chlorophyll pigments in plants absorb light. The difference in absorption between visible
and near-infrared light (longer than 0.7 µm) forms the basis for the measurement of
Normalized Difference Vegetation Index. (Graph courtesy Compton Tucker, NASA GSFC)